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Unome wrote:

sciencegirl03 wrote:This is what happened: We launched the helicopter with the rotor side down, it went down, started flipping, bumped into something, flipped over and then went straight up. We only had around 40 winds(with a 15:1 winder) so didn't fly for long, but looked good. Straight up and then down.

The other time, It went straight down when launched with rotors side down, but when we launched it rotors side up, it flew straight up. Again, there were around 40 winds. We will experiment with more winds soon. It is my first year doing this.

Could be the weight distribution. I did have to glue some broken parts. Will build a second one soon.

Your description, especially the part about it going down at first, sounds like you have the rotors pointing the wrong way. I'm fairly sure it's intended to fly with the rotors down (at least, based on the one device I've seen).

The rotors are supposed to be on the bottom. It sounds like you are winding the rubbers in the wrong direction so that when it's upside down it goes up.

Thanks, we will try winding in the opposite direction and post if we are successful.

sciencegirl03 wrote:Thanks, we will try winding in the opposite direction and post if we are successful.

Assuming that the rotors are counter rotating then you dont have to wind in the opposite direction just switch which rubber band you are putting one which rotor. One should rotate counter clockwise and the other clockwise.

Has anyone been able to build down to the competition weight of 3 grams?

I know David says in his instructions that they helicopters should turn out around 3.7 grams but I was wondering whether anyone had any tips to get down to minimal weight.

ScottMaurer19 wrote:

sciencegirl03 wrote:Thanks, we will try winding in the opposite direction and post if we are successful.

Assuming that the rotors are counter rotating then you dont have to wind in the opposite direction just switch which rubber band you are putting one which rotor. One should rotate counter clockwise and the other clockwise.

That did it! It is flying beautifully with the rotors down now!

Approximately how many winds should the rubber & motor stick be able to take, maximum? Let's say with a 1g motor of the thin rubber in the FFM kit.

SluffAndRuff wrote:Approximately how many winds should the rubber & motor stick be able to take, maximum? Let's say with a 1g motor of the thin rubber in the FFM kit.

Individual winds, perhaps 1000-1500 (i.e. 100-150 on a x10 winder). That's just a rough estimate though, I'm not sure of much more than that.

SluffAndRuff wrote:Approximately how many winds should the rubber & motor stick be able to take, maximum? Let's say with a 1g motor of the thin rubber in the FFM kit.

There's a handy break point equation out there:
break point = 42.2 * L * sqrt(L/W)
L = length in inches (EDIT: length of the loop when tied), W = weight in grams (in your case 1 gram, unless the motor also has rubber o-rings which weigh approximately 90 grams if you're using 2, so the actual rubber would be 0.9 grams)

Since the number you get is how many turns until the rubber breaks, multiply that by 0.9 to get approximately the amount of winds the motor can take. Remember to stretch wind (stretch out the motor 4-5 times it's original length, wind a bit more than half the amount of turns desired, then slowly move closer while winding till the end), otherwise the motor will break much more easily. Also keep in mind that the energy in the rubber diminishes after many uses.

I'm not sure how many winds the motor stick can take, you'll have to do some testing and see how many until it starts to bow.

JasperKota wrote:

SluffAndRuff wrote:Approximately how many winds should the rubber & motor stick be able to take, maximum? Let's say with a 1g motor of the thin rubber in the FFM kit.

There's a handy break point equation out there:
break point = 42.2 * L * sqrt(L/W)
L = length in inches (EDIT: length of the loop when tied), W = weight in grams (in your case 1 gram, unless the motor also has rubber o-rings which weigh approximately 90 grams if you're using 2, so the actual rubber would be 0.9 grams)

Since the number you get is how many turns until the rubber breaks, multiply that by 0.9 to get approximately the amount of winds the motor can take. Remember to stretch wind (stretch out the motor 4-5 times it's original length, wind a bit more than half the amount of turns desired, then slowly move closer while winding till the end), otherwise the motor will break much more easily. Also keep in mind that the energy in the rubber diminishes after many uses.

I'm not sure how many winds the motor stick can take, you'll have to do some testing and see how many until it starts to bow.

This formula is very rough and is different for each batch of rubber and quality of the stripper used to cut the rubber, so if you can't reach the number of winds given in the formula don't feel too bad. The rubber should also be stretched at LEAST 7 times is original length when winding, but stretching out even longer will help you pack in turns tighter, especially after you have wound the motor 3-5 times, which brings me to my next point:

Energy stored in motors does not decrease the more times you use it. In fact, the total energy capacity will increase. The peak torque of the motor may be lower, but you should be able to pack in more turns because the motor becomes stretched longer, and the average torque is approximately the same. Once the motor is used 3-4 times, the total energy capacity is substantially greater and can be more than a 10% gain than trying to use a fresh, unused motor.

If a competition was approaching, I would examine my motors that I used during practice after each wind for small cuts that resulted from winding, and the motors that would have very small cuts even after being used 3-4 times would be saved for competition as I knew they would have higher energy capacity than any new motor, and would still have a small probability of breaking. This would allow me to pack in turns without too much fear of breakage during the actual competition and I knew I would be flying with an optimal motor.

Rubber winding in itself is as much of an art and science as building and trimming the actual model, however it is still not too well understood. I have spoken to many world class free flight modelers and everyone has their own method for breaking in motors and winding them, so there is no right way in doing these things and there's still a lot of experimentation to be done.

Regarding Reveur's question about building to the 3.0 gram minimum:

This type of "build to weight" analysis has been posted previously (Jeff Anderson posted a thorough version), but here's a version specific to this year's Freedom Flight helicopters:

This year's FF kit motor sticks are about 2.5 square inches of 1/16" sheet each. If you are using 5.5 cubic foot density wood, which is a 9.8 gram sheet of 3"x36" wood (if consistent overall, which it won't be), the motor sticks will weigh about 0.234 grams each. If you are using 4.5 cu ft density wood, which is a very rare 8.0 gram sheet of 3"x36" wood, the motor sticks will weigh 0.188 grams each. The lighter wood will therefore reduce the helicopter overall weight by 0.092 grams, which is a 2.6% reduction in airframe weight if you are assuming a 3.5 gram helicopter. Total flying weight, of course should consider the weight of the rubber motors.

When building to weight is challenging, every component, every stick of wood and other parts, should be evaluated in this manner.

Another example: the 3k carbon tow used to reinforce the motor sticks weighs approximately 0.0075 grams per inch; times 18" on each stick (9" each side), this totals 0.135 grams. The CA glue used to apply the tow can double the total weight, as constructed. Practice applying the carbon tow to scrap wood, weighing parts and final assembly to learn to minimize the amount of glue used. Glue, especially CA glue, is pretty heavy. Also, maybe compare constructed weight using different lighter glue, like Duco Cement. Test on scraps to see if you feel that the strength/weight balance is appropriate.

Good luck,
Brian T.
Academy of Model Aeronautics member off and on since 1968